Method of stabilizing coating on aluminum



reamed Au 16, 1938 METHOD or smnmzmo COATING N ALUMINUM- Junius D. Edwards, Oakmont, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application June 3, 1935,

Serial No. 24,717

12 Claims.

This invention relates to a method of treating artificial oxide coatings formed on aluminum and aluminum base alloy surfaces and composed substantially of aluminum oxide, for the purpose of beneficially modifying the properties of such coatings, particularly with respect to their stability in use.

The oxide coatings contemplated by this invention may be formed artificially on aluminum and/or aluminum base alloy surfaces by various known chemical and electrochemical processes. The aluminum,-and by this term as used herein and in the appended claims is meant both aluminum and aluminum base alloys,-may, for example, be electrochemically oxidized by anodic treatment in an electrolytic cell containing as electrolyte a solution of an acid such as sulfuric acid, chromic acid, oxalic acid, or mixtures of these acids, or may be chemically oxidized without the use of electric current by simple immersion, for example, in a hot solution of an alkali carbonate containing a small amount of a dichromate. The coatings thus obtained are composed substantially of aluminum oxide, al-

though they contain generally a small amount of one or more other components as a result of the process by which they are produced. The oxide coatings will vary in their specific properties depending on the method of their production, but generally they have relatively good resistance to corrosion and abrasion, and they are more or less adsorptive, so. that they may be colored by means 0.1. dyes and inorganic pigments. These advantageous properties have led to the extensive adoption of oxide coatings for decorative purposes and for the protection of aluminum surfaces.

However, these oxide coatings are not completely stable, and upon long exposure, particu 40 larly in highly humid atmospheres, they show a tendency to become cloudy and-white. This instability and whitening appears to be the result of some action taking place within the oxide coating; it may be caused or accelerated by the presence in the coating of substances adsorbed from the electrolyte or chemical bath in which the coating was produced. The tendency of the coating to whiten varies in coatings produced in dif ferent solutions and is pronounced in connection with oxide coatings produced by anodic oxidation in sulfuric acid electrolytes. This whitening is objectionable in colored oxide coatings and is particularly objectionable in clear, transparent oxide coatings used as protective coatings on highly reflecting aluminum surfaces. Bright aluminum reflecting surfaces have found extensive application as a result of recently developed methods of applying clear, transparent oxide coatings as protective coatings to such surfaces. The whitening of such oxide coatings, if it proceeds only to a slight extent, reduces the utility of the reflecting surface materially.

' Another characteristic of some oxide coatings on aluminum is their tendency to craze or crack upon exposure to relatively high temperature. When an oxide-coated aluminum reflector is used in close proximity to a high wattage lamp, for example, the oxide coating may craze to a considerable extent, possibly because of the difference in thermal expansion of the oxide coating and that of the aluminum of the reflector. This crazing is generally scarcely visible but is nevertheless present and it may lower the protective value of the oxide coating.

It is an object of this invention to provide a method of treating oxide coatings on aluminum to so modify their properties and characteristics as to stabilize such coatings. It is more particularly an object of this invention to provide a method of treating oxide coatings on aluminum which will reduce the tendency of such coatings to become white and cloudy upon exposure to the weather and to condensed moisture in humid atmospheres. Another particular object of the invention is to increase the resistance to crazing and cracking of oxide coatings on aluminum surfaces, particularly aluminum reflecting surfaces, when exposed to relatively high temperatures.

This invention is based upon my discovery that when a preformed, adsorbent oxide coating on aluminum is treated with a solution of a phosphate, the phosphate is adsorbed in the coating and the properties of the oxide coating are materially modified without substantially changing its appearance. In carrying out the method of my invention, an aluminum article provided on its surface with an adsorbent oxide coating is subjected to the action of a phosphate solution, preferably hot, having a hydrogen ion concentration such that the solution does not appreciably attack the oxide coating at the temperature at which it is applied. By the treatment of oxide coatings with such solutions I have been able to produce coatings of improved stability which do not have so great a tendency to whiten upon exposure to the condensed moisture in humid atmospheres and which are resistant to crazing when heated to relatively high temperatures.

The phosphate solution used in treating an oxide coating according to my invention may be a solution of any soluble phosphate salt, or a mixture of phosphate salts, or a mixture of a phosphate with phosphoric acid. I have found it most desirable to use phosphate solutions which are neutral or slightly acid, and I prefer solutions having a pH of 4 to '7, although satisfactory results are also obtainable with phosphate solutions having a pH of about 3 to 8. For obtaining phosphate solutions of the proper hydrogen ion concentration I have found it particularly convenient to use solutions of the alkali metal salts of phosphoric acid, which class is understood to include the ammonium salts. In the specification and appended claims where the hydrogen ion concentration of a solution is specifled, the value given is the pH value of the solution measured at room temperature.

A wide range of alkalinity or acidity can be obtained by employing the primary, secondary or tertiary phosphates: of sodium, potassium and ammonium, or mixtures of these salts, or mixtures of these salts and phosphoric acid. The tertiary alkali metal phosphates, such as trisodium phosphate Na3PO4 and tripotassium phosphate K3PO4 are quite alkaline; a 1 percent solution of either salt has a pH greater than 9. It is generally-necessary, therefore, when using solutions of these salts, to adjust the hydrogen ion concentration of the solution to bring it within the desired range. This may be done by any suitable means, such as the addition of a suitable amount of phosphoric acid or acid phosphate salts. The secondary alkali metal phosphates such as disodium hydrogen phosphate or dipotassium hydrogen phosphate, produce slightly alkaline solutions which may in certain cases be used without adjustment of the hydrogen ion concentration to obtain satisfactory results. In most cases, however, it is desirable to use these salts in mixtures with phosphoric acid or the primary alkali metal phosphate salts to obtain solutions having hydrogen ion'concentrations between pH 4 and pH 7. The primary alkali metal phosphates, such as sodium dihydrogen phosphate or potassium dihydrogen phosphate, yield solutions having hydrogen ion concentrations within this preferred range without adjustment and I prefer in most cases to use solutions of these salts alone.

In general the particular alkali metal ion or ions present in the solution will not substantially modify the results obtained, and good results may be obtained with solutions of potassium, sodium or ammonium salts having hydrogen ion concentrations within the range of pH 4 to '7. However, I have found that it is generally desirable, when using solutions of ammonium salts to use solutions of somewhat lower hydrogen ion concentration than would be desirable with potassium or sodium salts to produce comparable results.

The concentration of the phosphate solution is not critical as long as the hydrogen ion concentration is maintained within the range of pH 3 to 8. I prefer in most cases to use a 1 to 5 per cent solution of an alkali metal phosphate, but higher or lower concentrations are effective and can be used where desired. The solution may be used repeatedly without substantial deterioration and in some cases slightly improved results have been obtained with solutions after repeated use.

The temperature of the phosphate solution may vary from room temperature to boiling with substantial improvement in the stability of the oxide coating treated. I have found, however, that by the use of hot solutions, a greater increase in the stability of the coating is obtained. I prefer, therefore, to treat the oxide coating with a phosphate solution at a temperature within the range of about 75 to 100 C. The time of treatment will vary with the type of oxide coating being treated and with the concentration and temperature of the phosphate solution. In general, treatment at 75 to 100 C. for 15 to 30 minutes will produce satisfactory results, although longer or shorter treatment may be found desirable or necessary in certain cases.

The increase in stability, with respect to whitening upon exposure in a humid atmosphere, obtainable by the application of my method is shown by the following results obtained in connection with oxide-coated bright aluminum reflector surfaces. A bright aluminum reflecting surface was obtained by electrolytic treatment in a fluoborate solution as described in the copending application of Ralph B. Mason, Serial No. 683,344, flled August 2, 1933, and was provided with a clear, transparent oxide coating by anodic oxidation in a 15 per cent sulfuric acid electrolyte. The oxidecoated reflecting surface thus obtained was rinsed to remove electrolyte and immersed in a boiling 1 per cent solution of primary potassium phosphate for 15 minutes. The bright surface thus treated showed no evidence of dulling or attack. A second aluminum reflector electrolytically brightened and oxide-coated in sulfuric acid as above described was immersed in boiling water for 15 minutes. A-slight scum was produced on the surface but was readily removed by light polishing. These reflectors were exposed together in an atmosphere substantially saturated with water vapor at a temperature of 45 C. for a period of 15 days. During the exposure the reflectors rested on a cold plate so that moisture from the atmosphere was freely condensed on their surfaces. At the end of this test the oxide coating which had been treated with a phosphate solution showed no substantial whitening, while the coating which had been boiled in water only showed white stains and spots which could not be removed even by light polishing.

The improved resistance to crazing and cracking on exposure to high temperatures of oxide coatings treated with phosphate solutions according to this invention is demonstrated by the following results. Two samples of aluminum were oxide-coated by anodic oxidation in sulfuric acid. One sample was immersed for 15 minutes in a boiling 2 per cent solution of primary potassium phosphate (KH2PO4) and the other sample was immersed in Water at C. for 15 minutes. These two oxide-coated samples were then heated in an electric furnace at a temperature of 340 C. for-about 10 /2 hours. Upon examination the oxide coating which had been treated with phosphate solution showed a few small isolated cracks. The oxide coating which had been treated in water showed many long, continuous cracks over its entire surface.

Having now particularly described the method of my invention and the improved results obtained thereby, what I claim is:

1. A method of stabilizing an oxide-coated aluminum surface without substantially modifying its appearance, comprising subjecting said oxide coating to the action of an alkali metal phosphate solution having a pH of about 3 to 8.

2. A method of stabilizing an oxide-coated aluminum surface without substantially modifying its appearance, comprising subjecting said oxide coating at a temperature between about 75 and 100 C. to the action of an alkali metal phosphate solution having a pH of about '3 to 8.

3. A method of stabilizing an oxide-coated aluminum surface without substantially modifying its appearance, comprising subjecting said oxide coating to the action of an alkali metal phosphate solution having a pH of 4 to '7.

4. A method of stabilizing an oxide-coated aluminum surface without substantially modifying its appearance, comprising subjecting said oxide coating at a temperature between about 75 and 100 C. to the action of an alkali metal phosphate solution having a pH of 4 to 7.

5. A method of stabilizing an oxide-coated aluminum surface without substantially modifying its appearance, comprising subjecting said oxide coating to the action of a primary alkali metal phosphate solution.

6. A method of stabilizing an oxide-coated aluminum surface without substantially modifying its appearance, comprising subjecting said oxide coating at a temperature between about 75 and 100 C. to the action of a primary alkali metal phosphate solution.

'7. A method of stabilizing an oxide coating produced on an aluminum surface by anodic oxidation in a sulfuric acid electrolyte without substantially modifying the appearance of said oxide coating comprising subjecting said oxide coated surface to the action of a phosphate solution having a pH of about 3 to 8.

8. A method of stabilizing an oxide coating produced on an aluminum surface by anodic oxidation, without substantially modifying the ape pearance of said oxide coating, comprising subjecting said oxide-coated surface to the action of an alkali metal phosphate solution having a pH of about 3 to 8. v

9. A method of producing on an aluminum surface a stabilized oxide coating, without substantially modifying the appearance of said oxide coating, comprising electrolytically oxidizing said surface and subjecting the coating thus formed to the action of an alkali metal phosphate solution having a pH of about 3 to 8.

10. A method of stabilizing an oxide coating produced on an aluminum surface by anodic oxidation, without substantially modifying the appearance of said oxide coating, comprising subjecting the said oxide coating, at a temperature between about 75 and 100 C., to the action of an alkali metal phosphate solution having a pH of about 3 to 8.

11. A method of stabilizing an oxide coating produced on an aluminum surface by anodic oxidation, without substantially modifying the appearance of said oxide coating, comprising subjecting the said oxide coating to the action of an alkali metal phosphate solution having a pH of about 4 to '7.

12. A method of stabilizing an oxide coating produced on an aluminum surface by anodic oxidation, without substantially modifying the appearance of said oxide coating, comprising subjecting the said oxide coating, at a temperature between about 75 and 100 C., to the action of an alkali metal phosphate solution having a pH of about 4 to 7.

JUNIU S D. EDWARDS. 

